Conventionally, in biomeasurement for detecting proteins or DNAs, there has been known a surface plasmon resonance fluorescence analysis method (Surface Plasmon-field enhanced Fluorescence Spectroscopy: SPFS), as a method for detecting a specimen (material to be detected) with high sensitivity.
According to SPFS, a prism having a metal film made of e.g. gold or silver formed on a predetermined surface thereof is prepared, excitation light is caused to enter the metal film from the side of the prism in such a manner that the predetermined surface is subjected to total reflection, and light (evanescent wave) emerging from the metal film in reflecting the excitation light on the metal film is utilized. Specifically, according to SPFS, when the metal film is subjected to total reflection of excitation light, a specimen that is contained in a sample solution and flows over the surface of the metal film, or a fluorescent material (labeling material) labeled on the specimen is excited by an evanescent wave emerging from the metal film, and fluorescence (excitation fluorescence) emitted from the specimen or from the fluorescent material is analyzed. By performing the analysis as described above, it is possible to detect the presence of the specimen or to detect the quantity of the specimen.
In an optical measurement utilizing SPFS, it is necessary to set an excitation light incident angle (specifically, an incident angle of excitation light with respect to a metal film) in accordance with individual prisms or conditions of reaction fields for sufficiently enhancing the electric field near the surface of the metal film by surface plasmon resonance. In view of the above, in the optical measurement utilizing SPFS, generally, a regression measurement system utilizing light resulting from an electric field (enhanced electric field) which is enhanced near the surface of the metal film is configured for adjusting the incident angle of excitation light with respect to the metal film. In such an optical measurement, excitation light is caused to enter the prism with a small angle with respect to the metal film so that the excitation light enters while undergoing total reflection in the interface between the metal film and the prism. As a result of the above operation, the reflection position (irradiation position) of excitation light on the reflection film is likely to displace, resulting from slight displacement of the incident angle of the excitation light with respect to the metal film. Further, the reflection position may displace afterwards due to a temperature change in a measurement environment. Since the region of electric field enhanced by surface plasmon resonance is narrow, such displacement of the reflection position of excitation light may result in lowering the precision in measuring light resulting from the enhanced electric field.
In view of the above, as disclosed in patent literature 1, there has been developed an apparatus, wherein the entirety of an excitation optical system is made to swing by a link mechanism in such a manner that the reflection position (irradiation position) of excitation light on a metal film formed on a prism coincides with a center of rotation, whereby the incident angle of excitation light is made to be adjustable. With use of the apparatus, swinging the entirety of the excitation optical system about the reflection position as a center of rotation makes it possible to adjust the irradiation direction of excitation light of the excitation optical system for securing an incident angle that enables to obtain an enhanced electric field suitable for detecting a specimen, while suppressing displacement of the reflection position.
Further, as disclosed in patent literature 2, there has been developed an apparatus, wherein multiple excitation lights having different incident angles are simultaneously irradiated at a reflection position (irradiation position) of excitation light on a metal film. With use of the apparatus, one of the excitation lights has an incident angle suitable for individual prisms or for individual conditions of reaction fields. Thus, it is possible to obtain an enhanced electric field suitable for detecting a specimen without causing displacement of the reflection position.
However, in the former apparatus, since the entirety of the excitation optical system provided with a light source and lenses is made to swing by a link mechanism, the excitation optical system is likely to vibrate due to the weight thereof. Further, since the link mechanism is constituted of many components, the reflection position of excitation light on a metal film may displace, resulting from backlash of each of the components.
Further, in the latter apparatus, it is possible to suppress displacement of the reflection position, because scanning of the incident angle of excitation light with respect to a metal film is not performed by a mechanism or a like device. However, self fluorescence is generated in the course of propagation of excitation light that does not contribute to surface plasmon resonance on the metal film through a prism. Then, in measuring fluorescence generated by excitation of a fluorescent material labeled on a specimen in an enhanced electric field, the self fluorescence of excitation light that does not contribute to surface plasmon resonance is also measured. This may lower the S/N ratio of a signal to be obtained by the measurement.